1. Field of the Invention
The present invention relates to an apparatus and a method for measuring a cured state of a reaction curable resin, which changes from liquid to solid by causing a chemical reaction when stimulated by light and heat. Particularly, the present invention relates to a measurement of a temporal change in the cured state.
2. Related Background Art
A reaction curable resin has been frequently used in fields of ink, paint, plastic-material coating and lens (glass lens). Particularly, it is known that a replica forming method using an ultraviolet curable resin is suitable as a process of forming an optical device having a miniature shape, such as a diffractive optical device.
In a replica formation method, an optical device is formed by dripping an ultraviolet curable resin on the previously polished lens surface to give the resin an aspherical shape using a mold and then applying ultraviolet radiation to cure a resultant aspherical layer, and releasing the molded item from the mold. However, the replica forming method, has a problem in that the resulting object may have a defective appearance due to the fluctuation in the cured state of an ultraviolet curable resin. Moreover, because curing excessively advances, the resin becomes more fragile and the optical device is chipped when it is released from the mold or, alternatively, the optical device having a desired shape cannot be obtained because it is released from the mold before being sufficiently cured. Therefore, determining optimum curing conditions by analyzing the cured state of the ultraviolet curable resin and identifying a factor causing the problem is very important.
As a method for analyzing and measuring a cured state of a reaction curable resin, infrared light spectrometry, such as FT-IR (Fourier transformation infrared spectral analysis), is generally used, as disclosed in Japanese Patent Application Laid-Open No. H06-294734. For measuring the cured state of a resin using FT-IR, it is known that any one of a photo-curing resin, heat curing resin and two-component system resin can be used. Moreover, to determine the temporal changes in the cured state of an ultraviolet curable resin, a method using an infrared spectrum peak is disclosed in Japanese Patent Application Laid-Open No. 2000-055806. The infrared spectrum peak detects changes in the molecular structures following the curing reaction of an ultraviolet curable resin and measures a change amount of peak intensities according to curing.
Generally, when curing a reaction curable resin, the whole resin is not uniformly cured. A distribution necessarily occurs in curing rates and there are irregularities in the cured states. Some of the factors that cause this distribution to occur include the following: a different position or angle of different portions of the resin to a light source or heat source, a difference in anaerobic characteristics of a resin due to a variance in the shape of the resin, resulting in different curing reaction rates in different portions of the surface of the resin, and the curing reaction rate in the interior portion of a resin differs from the outer portions, depending on the thickness of each portion of a resin. Therefore, a measuring method capable of measuring a change in cured states of the resin as a whole following the passage of time and displaying the change as a screen image is necessary.
However, the method using infrared spectroscopic analysis shown in Japanese Patent Application Laid-Open No. H06-294734 and the method using an infrared spectrum peak disclosed in Japanese Patent Application Laid-Open No. 2000-055806 are methods for respectively measuring the cured state at a single point in an object. Thus, it is impossible to use these methods to capture the cured state of the entire reaction curable resin as a screen image. In the case of these measuring methods using infrared light, the heat due to the infrared light propagates to a reaction curable resin. Therefore, in the case of a thermal curing resin whose curing reaction progresses due to heat, there is a problem in that the curing reaction of the resin is influenced by the heat and an accurate reaction state cannot be measured. Moreover, when measuring an optical device- produced by the replica forming method, a reaction curable resin is cured while it is held between a lens and a mold. Therefore, to measure the cured state of the reaction curable resin, it is necessary to measure the cured state through the lens. However, it is impossible to measure the cured state, because infrared light does not pass through the glass materials or plastic materials, which are used as the lens material.